Polymeric pigments and method for preparation thereof

ABSTRACT

Improved polymeric pigments for use in paper coatings are provided by an emulsion polymerization process comprising the steps of (1) emulsion polymerizing at least 50 weight percent of total monomer in absence of emulsifier, (2) adding an emulsifier and (3) continuing emulsion polymerization of the remaining monomer charge.

United States Patent [191 Loeffler et al.

[ POLYMERIC PIGMENTS AND METHOD FOR PREPARATION THEREOF [75] Inventors:Norman Raymond Loeffler,

Freeport; Hershel Beebe Prindle, Jackson, both of Tex.

[73] Assignee: The Dow Chemical, Midland, Mich.

[22] Filed: Sept. 2, 1971 [21] Appl. No.: 177,431

[52] US. Cl.260/29.6 TA, 117/155 UA, 260/29.6 T, 260/29.6 H, 260/29.7 H,260/29.7 T, 260/41 R, 260/785 BB, 260/80.7, 260/808 [51] Int. Cl. C08145/04, D21h 1/28 [58] Field of Search... -260/29.6 TA, 29.7 T, 29.7 H,260/29.6 H; 117/155 UA [56] References Cited UNITED STATES PATENTS2,837,444 6/1958 Hahn 260/29.6 TA

[ June 25, 1974 3,057,812 10/1962 Straughan et al. 260/29.6 TA 3,265,6548/1966 Glabisch et a1 260/29.6 TA 3,297,614 1/1967 Pueschner et al.260/29.6 TA 3,501,432 3/1970 Wright et a1. 260/29.6 TA 3,513,120 5/1970Pohlemann et a1 260/29.6 TA 3.595,823 7/1971 Huang 260/29.6 TA 3,634,2981/1972 Wamsley et a1 260/296 TA Primary Examiner-Lucille M. PhynesAttorney, Agent, or FirmR. G. Waterman; M. S.

Jenkins [5 7] ABSTRACT Improved polymeric pigments for use in papercoatings are provided by an emulsion polymerization process comprisingthe steps of (1) emulsion polymerizing at least 50 weight percent oftotal monomer in absence of emulsifier, (2) adding an emulsifier and (3)continuing emulsion polymerization of the remaining monomer charge.

13 Claims, N0 Drawings POLYMERIC PIGMENTS AND METHOD FOR PREPARATIONTHEREOF BACKGROUND OF THE INVENTION This invention relates to polymericpigments for lightweight paper coatings having high opacity and visualgloss and to a method for the preparation thereof.

Most paper surfaces require a coating in order to have good printingqualities and sufficient opacity. Conventional paper coatings containinorganic pigments such as kaolin clay or titanium dioxide to give thecoated paper substrate the opacity required. However, the inorganicpigments substantially increase the coating weight which in turnincreases the cost of mailing substrates so coated.

Recently, lightweight paper coatings containing as pigment, discretepolymeric particles which are insoluble in water and the binder of thecoating and which retain their discrete character and particle size of0.3 to 0.8 micron during coating operation have been disclosed. SeeBelgian Pat. No. 733,548.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a novel, improved polymeric pigment and a method for thepreparation thereof.

In one aspect, the present invention is an aqueous dispersion of thepolymeric pigment. The polymeric pigment comprises discrete particles ofnon-film forming, water-insoluble polymer of emulsion polymerizablemonomer. The aqueous dispersion of polymeric pigment is prepared by themethod described hereinafter. The discrete particles which comprise thepolymeric pigment of this invention have an average diameter in therange from about 0.3 to about l.0 micron with no more than about weightpercent of the particles based on total weight of the discrete polymericparticles of the polymeric pigment having diameters outside of saidrange.

In a second aspect, this invention is a method for pre-. paring theaqueous dispersion of polymer pigment. This method is a modifiedemulsion polymerization process comprising the steps of l subjecting apolymerization recipe containing an aqueous medium, a catalyst and anincrementally-added initial portion of total emulsion polymerizablemonomer to conditions of emulsion polymerization essentially in absenceof emulsifying agent, said initial portion of monomer constituting atleast about 50 weight percent up to and including 99.9 weight percent oftotal monomer; (2) subsequently adding to the polymerization recipe fromabout 0.25 to about weight percent of emulsifying agent based on totalmonomer; and (3) continuing addition and emulsion polymerization ofremaining portion of total monomer.

As a third aspect of this invention, there is provided an improved papercoating composition comprising from about L3 to about 1 L5 parts byvolume of binder per about 38.8 parts by volume of a pigment, at least aportion of which is the aforementioned polymeric pigment which issubstantially insoluble in the binder under conditions normally employedin paper coating processes. Said portion of polymeric pigment issufficient to impart lightweight and/or improved properties such asopacity, brightness, gloss, and printing qualities to dried coatings ofsaid paper coating compositions.

As a fourth aspect of this invention, there is provided a coated paperarticle comprising a paper substrate having intimately adhered thereto adried coating of the aforementioned composition wherein the polymericpigment essentially retains its original discrete character and originalparticle size.

The practice of this invention utilizing at least an effective amount ofthe polymeric pigment produces a coated substrate which has a level,unpocked surface and a considerably decreased coating weight. Suchcoated substrates possess measurably improved brightness, opacity,gloss, and printing qualities as compared to the same properties of asubstrate having an adherent coating of binder and non-film formingpolymeric particles prepared by conventional emulsion polymerizationprocesses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The discrete particles usefulas the polymeric pigment are composed of any non-film forming organicpolymer which is water-insoluble and is insoluble in the particularbinder to be used in the coating composition. Preferred polymers arethermoplastic, organic, resinous materials which are substantiallycolorless.

By non-film fonning, it is meant that the dispersed polymeric pigmentdoes not coalesce to form a film at ambient temperature and attemperaturesselected to dry or finish the coated article. While suchtemperature requirement varies with the type of finishing method used,it is preferred that the polymer in the form of the discrete particlesnot be film forming at temperatures of l40 F or less if the coatedsurface is to be finished by processes such as calendering or supercalendering. If the discrete polymeric particles throughout the coatingare permitted to fuse or coalesce during the coating process, the lightscattering properties (opacity) of the coated surface will be reducedsubstantially. Accordingly, polymers preferred for use as the polymericpigment have a Vicat Softening Point, as defined and determined by ASTMD-l 525-65-T, of greater than about 140 F. For the same reasons, it isalso necessary to use polymeric particles which are not dissolved orsoftened by the particular binder chosen.

In preparation of the polymeric pigment, any monomer or mixture ofmonomers can be used which is polymerizable under conditions of aqueousemulsion poly merization and which forms a polymer having the specifiedphysical properties of being water-insoluble and non-film forming.Preferred emulsion polymerizable monomers which can be polymerizedand/or copolymerized with each other in any proportions and/or withother monomers as specified hereinafter to yield such polymers includeethylenically unsaturated monomers such as the monovinylidene carbocylicaromatic monomers, e.g., styrene, amethylstyrene, ar-(t butyl)styrene,ar-methylstyrene, ar,ar-dimethylstyrene, ar-chlorostyrene,ar-(t-amyl)styrene, ar-bromostyrene, ar-fluorostyrene, ar-cyanostyrene,ar-methoxystyrene, ar-ethylstyrene, ar-hydroxymethylstyrene,ethoxystyrene, ar-chloro-ar-methylstyrene, ar,ardichlorostyrene,ar,ar-ditluorostyrene, vinyl naphthalene, and other such emulsionpolymerizable mono mers having not more than 26 carbon atoms; esters ofa,B-ethylenically unsaturated carboxylic acids which polymerize to formnon-film forming polymers, e.g., methyl methacrylate, chloroethylmethacrylate, 2-butyl methacrylate, 3,3-dimethylbutyl methacrylate, 3,3-dimethyl-2-butyl methacrylate, ethyl methacrylate, iso butylmethacrylate, isopropyl methacrylate, phenyl methacrylate, butylchloroacrylate, cyclohexyl chloroacrylate, ethyl chloroacrylate, methylchloroacrylate, isopropyl chloroacrylate and other such esters whereinthe alkyl moiety has from 1 to 20 carbon atoms and the acid moiety hasfrom 3 to 8 carbon atoms; a,,B-eth'ylenically unsaturated esters ofnon-polymerizable carboxylic acids, e.g., vinyl benzoate, vinylar-toluate, vinyl ar-ethylbenzoate, allyl ar-ethylbenzoate, vinyltrimethylacetate, vinyl pivilate, vinyl trichloroacetate and other suchmonomers wherein the unsaturated moiety has from 2 to 14 carbon atomsand the acid moiety has from 2 to 12 carbon atoms; a,B-ethylenicallyunsaturated nitriles, e.g., acrylontrile, methacrylonitrile,fumaronitrile and other such nitriles having not more than 12 carbonatoms; other polymerizable vinyl monomers such as vinyl chloride, vinylbromide and the like.

The foregoing monomers are generally classified as hard monomers as theypolymerize or copolymerize with each other to form non-film formingpolymers as required in the practice of this invention.

Lesser amounts, such as less than about 45 weight percent based on thepolymer, of other ethylenically unsaturated monomers which normallypolymerize to form film-forming polymers (so-called soft monomers) aresuitably copolymerized with the foregoing hard monomers. Examples ofsuch monomers include conjugated aliphatic dienes such as 1,3-butadiene,isoprene, 2-chloro-l,3-butadiene and other such dienes having not morethan 14 carbon atoms; alkyl acrylates such as methyl acrylate, ethylacrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, amylacrylate, lauryl acrylate, isoamyl acrylate, Z-ethylhexyl acrylate,octyl acrylate, and other such acrylates having alkyl moieties of notmore than 18 carbon atoms; unsaturated esters of saturated carboxylicacids such as vinyl acetate, vinyl propionate, vinyl butyrate, allylacetate and other such esters having not more than 18 carbon atoms;esters and half esters of B-ethylenically unsaturated polycarboxylicacids, e.g., dimcthyl fu'maratc, diethyl maleate, methyl ethyl fumarate,ethyl hydrogen maleate, dioctyl fumarate and the like; othercopolymerizable vinyl monomers containing a single polymerizableethylenically unsaturated group such as vinyl fluoride, vinylidenechloride and vinylidene fluoride. Maximum concentrations of thesemonomers are governed primarily by the temperature to be reached by thecoating during the coating process and the degree to which a particularmonomer lowers the softening point of the resulting copolymer. Forillustration, if a copolymer of styrene and butadiene is to be used asthe polymeric pigment, budiene normally is not present in the copolymerin amount more than about 20 weight percent. If, however, thestyrene/butadiene copolymer has more than the normal amount ofcrosslinking, butadiene may be present in concentration greater than 20weight percent with the maximum concentration of butadiene beingdependent on the actual degree of crosslinking. Increased crosslinkingis usually promoted by irradiation or by use of a suitable crosslinkingagent such as unsaturated polyester or polyethylenically unsaturatedmonomer. Exemplary polyethylenically unsaturated monomers includedivinyl benzene, trivinyl benzene, divinyl naphthalene, and the like. Inregard to the use of the aforementioned soft monomers, use in anyconcentration is suitable provided that the resultant polymer isnon-film forming as required in the practice of this invention.

In addition to the foregoing monomers, other monomers which may also becopolymerized constituents of the polymeric pigment area,B-ethylenically unsaturated carboxylic acids including both monoandpoly carboxylic, e.g., dicarboxylic, acids, such as acrylic acid,methacrylic acid, itaconic acid, fumaric acid, maleic acid andanhydride, citraconic acid and anhydride and other such acids. Maximumconcentrations of these acid comonomers in the polymeric pigment arelimited by the degree to which they promote water solubility of thepolymer. Since the polymeric pigment is required to be water-insoluble,the acid comonomers are generally employed in concentrations not greaterthan 25 weight percent of the polymeric pigment.

Preferred polymers used in the polymeric pigment are copolymers of fromabout to about 99 weight percent of one or more of the aforementionedhard monomers, from about 0 to about weight percent of one or more ofthe aforementioned soft monomers and from about 1 to about 15 weightpercent of one or more emulsion copolymerizable a,-B-ethylenicallyunsaturated carboxylic acids, preferably those having 3 to 8. carbonatoms. Especially preferred copolymers are copolymers of from about toabout 99 weight percent of monovinylidene carbocylic aromatic monomerssuch as styrene and ar-(t-butyl) styrene, from about 0 to about 49weight percent of a,B-ethylenically unsaturated nitrile such asacrylonitrile and methacrylonitrile, and from about 1 to about 5 weightpercent of a,B-ethylenically unsaturated carboxylic acid such as acrylicacid, methacrylic acid and itaconic acid. Examples of such especiallypreferred copolymers are styrene/acrylic acid copolymers,styrene/acrylonitrile/itaconic acid copolymers, styrene/methacrylic acidcopolymers, vinyl benzoate/acrylic acid copolymers, t-butylstyrene/acrylonitrile/acrylic acid copolymers and vinyl chloride/acrylicacid copolymers. In the foregoing preferred copolymers, it is sometimesbeneficial to copolymerize from about 1 to about 15 weight percent ofpolyethylenically unsaturated monomer such as divinyl benzene therewith.

The specific gravity of the preferred polymers used in the practice ofthe invention usually is within the range of about 0.9 to about 1.4g/cc. In order that the coating be considerably lighter in weight thancoatings containing inorganic pigments, it is preferable that theparticles be made from a polymer which is predominantly hydrocarbonsince such polymers have a specific gravity generally less than about1.4. The refractive index of preferred polymers is generally in therange of from about 1.2 to about 1.7, especially from about 1.4 to about1.6.

In addition to being composed of discrete particles of non-film formingpolymer as set forth hereinbefore, it is critical that the polymericpigment be in the form of particles having an average diameter in therange of from about 0.3 to about 1.0 micron, with no more than about 5weight percent of the particles based on total weight of particlesforming the polymeric pigment having diameters outside said range,preferably not more than 3 weight percent. The method described hereinis critical to produce such polymeric particles. Moreover, such methodusually produces particles which have essentially rough surfaces asobserved with an electron transmission microscope. Aqueous dispersionsof these rough surface particles generally provide better coatings.Also, the method often produces bimodal, or even trimodal, aqueousdispersions of polymeric particles wherein most of the polymer is in theform of particles having diameters in the range specified hereinbeforeand a minor amount, e.g., not more than about 5 percent, is in the formof particles having diameters in the range of from about 0.03 to about0.15 micron. In the practice of the method, it has been observed thatfor any given monomer system an aqueous dispersion of polymericparticles having improved coating properties is always produced ascompared to aqueous dispersions made using a conventional emulsionpolymerization process.

The polymeric pigment as described hereinbefore is advantageouslyprepared by an improved emulsion polymerization process as broadly setforth hereinbefore. More specifically, the improved emulsionpolymerization process of this invention is preferably prepared in abatchwise manner indicated as follows. The polymerization zone ispreferably purged with inert gas, charged with a water-solublepolymerization catalyst and water, and brought to a suitable reactiontemperature. The amounts of water and catalyst initially charged to thepolymerization zone are those typically employed in conventionalemulsion polymerization carried out by continuous feeding of monomerduring polymerization. Preferably, the amount of catalyst charged isfrom about 0.] to about 1 weight percent based on total monomer and theamount of water charged is from about 50 to about 150 weight percentbased on total monomer.

It is understood, however, that it is within the scope of this improvedprocess to charge the polymerization zone with additional amounts ofcatalyst and water during polymerization, in fact, it is sometimesadvantageous to do so and also to add basic materials such as ammoniumhydroxide or alkali metal hydroxide in order to enhance stability of thelatex. It is further understood that catalyst can be added continuously,separately or with the monomer feed stream. In addition, activators suchas a sulfoxylate activator can be added intermittantly or continuouslyto the polymerization zone in so-called redoxcatalyzed" polymerization.

As polymerization catalysts, there may be used one or more peroxideswhich are known to act as freeradical catalysts. Usually convenient arethe persulfates (including ammonium, sodium, and potassium persulfates),hydrogen peroxide, or the perborates or percarbonates. There may also beused organic peroxides, either alone or in addition to inorganicperoxide or sulfoxylate compounds. Typical organic peroxides includebenzoyl peroxide, tert-butyl hydroperoxide, cumene peroxide, acetylperoxide, caproyl peroxide, tert-butyl perbenzoate, tert-butyldiperphthalate, methyl ethyl ketone peroxide, and the like.

The choice of an inorganic or organic peroxide catalyst depends in partupon the particular combination of monomers to be polymerized. As mightbe expected, some of the monomers respond better to one type of catalystthan the other. The usual amount of catalyst required is approximatelyfrom about 0.01 percent to about 3.0 percent by weight as based on theweight of the total monomer charge.

In some instances, in order to effect polymerization at a temperaturebelow that at which coagulation of the latex might occur, it may bedesirable to activate the catalyst. The activation may be bestaccomplished by using a redox system in which a reducing agent withinthe limits of about 0.001 percent to about 6 percent as based on theweight of total monomers is present in addition to the peroxidecatalyst. Many examples of such redox systems are known. Agents, such ashydrazine or a soluble oxidizable sulfoxy compound, including the alkalimetal salts of hydrosulfites, thiosulfates, sulfites, and bisulfites,and the like can be employed. Redox systems may be activated by thepresence of a small amount (a few parts per million) of polyvalent metalions. Ferrous ions are commonly and effectively used or a tertiary aminewhich is soluble in the reaction medium may also be used as anactivator.

Continuous feeding of the initial portion of total monomer is begunafter water and catalyst have been charged or simultaneous therewith.The initial monomer portion constitutes from about 50 to about 99.9weight percent of total monomer charge, preferably from about to about85 weight percent. The polymerization zone is brought to desiredpolymerization temperature which is advantageously in the range of fromabout 30 to about 98 C, preferably from about to about 98 C, prior tobeginning of monomer feed or shortly thereafter. Generally, aqueousdispersions of polymeric particles having better coating properties areproduced at the higher temperatures within the aforementioned range. Therate of monomer feed is not particularly critical; however, it isgenerally desirable to feed at a rate such that addition of totalmonomer charge will be complete in a period from about 2 to about 24hours, preferably from about 4 to about 6 hours. It is advantageous, butnot critical, to stop the monomer feed after about 7 to about 15 weightpercent of the total monomer has been added and the mixture is allowedto digest for about 5 to about 60 minutes before resuming addition ofthe initial portion of the total monomer charge. This digestion periodwill hereinafter be referred to as an initial stage digestion. Suchpractice usually provides a very mild exotherm in the polymerization,which otherwise, with some monomers, will occur at an unpredictable timeand be quite vigorous. Surprisingly, products produced by such practicehave even better coating properties than the improved products otherwiseobtained. Employment of this advantageous step also reduces the amountof coagulum byproduct which is often produced.

It is critical in the practice of the method of this invention thatpolyermization of the initial portion of total monomer be carried outessentially in the absence of emulsifying agent, i.e., in the presenceof very little or no conventional external emulsifying agent. Traceamounts of emulsifying agent such as less than about 0.015 weightpercent based on total monomer, and preferably from about 0.0001 toabout 0.01 weight percent, may be present during polymerization of theinitial portion of total monomer without adversely affecting theproduct. In fact, for certain monomers, such practice produces productshaving even better coating properties than the improved productsotherwise ob tained. For the purpose of this invention, the termemulsifying agent" includes emulsifiers as defined hereinafter which areconventionally employed in emulsion polymerization, and specificallyexcludes polyermization catalysts and monomeric materials as definedhereinbefore.

During the initial, substantially emulsifier-free stage of thepolymerization reaction, it is preferable to permit polymerization toproceed without addition of further ingredients to the polymerizationother than addition of catalyst and, if desired, base and/or activator,for a period of from about 1 to about 8 hours to avoid buildup ofreactants. After the initial portion of monomer has been added andpolymerization thereof is essentially complete, emulsifying agent isadded to the polymerization recipe. As a general rule, from about 0.25to about 10 weight percent of emulsifying agent based on total monomeris an advantageous amount, with from about 0.5 to about 4 weight percentbeing preferred. It is especially preferred to add an excess ofemulsifying agent. By "an excess" of emulsifying agent is meant anamount more than that required to completely cover the surface areas ofthe latex polymer particles such that new polymerization sites areprovided. Relationship of surface area of latex particles to emulsifierconcentration is set forth in D. C. Blackley, High Polymer Latices, Vol.2, 486-491 (1966). The amounts of emulsifiers required to provide anexcess depend primarily on the concentration of monomers to be handledand, to a further extent, with the choice of emulsifiers, monomers andproportions of monomers.

Emulsifying agents suitable for the purposes of this invention are thewater-soluble anionic and non-ionic surfactants, with anionic ormixtures of anionic and non-ionic being preferred. Non-ionic surfactantswhich are suitable include the polyoxyalkylene agents, e.g.,polyethyleneoxyethanol derivatives of methylene linked alkyl phenols,the ethylene glycol polyethers, the alkyl phenoxypolyethyleneoxyethanols having alkyl groups of 7 to 12 carbon atoms suchas nonylphenoxypoly(ethyleneoxy)ethanols and condensation products ofethylene oxide with high alkyl mercaptans having alkyl groups of about 9carbon atoms, and condensation products of ethylene oxide with alkylthiophenols having alkyl groups of 6 to 15 carbon atoms; ethylene nonylphenol polyethers; the fatty acid esters of polyhydric alcohols, e.g.,propylene glycol fatty acid ester; and others set forth in Becher,Emulsions: Theory and Practice, 2nd Ed., Reinhold PublishingCorporation, New York, 221-225 (1965). Anionic surfactants include watersoluble soaps of soap-forming monocarboxylic acids, e.g., alkali metalsalts of linoleic acid dimer; and sulfated and sulfonated compoundshaving the general formula R O $0 M and R SO M, wherein R represents anorganic radical having from 9 to 23 carbon atoms and M represents analkali metal, an ammonium or amine group. Examples of the sulfonate andsulfate emulsifiers include sodium dodecyl benzene sulfonate, sodiumoleyl sulfate, ammonium dodecyl benzene sulfonate, potassium laurylsulfate, sodium dodecyl diphenyl oxide disulfonate, dioctyl potassiumsulfosuccinate, dihexyl sodium sulfosuccinate, and the arylsulfonate-formaldehyde condensation products.

Following addition of the emulsifying agent, the continuous addition ofremaining monomer charge to the polymerization zone is resumed .andpolymerization under similar conditions of time, temperature andcatalyst set forth herein is continued. During the period of additionand polymerization of remaining monomer, it is sometimes desirable tocharge the polymerization zone continuously or intermittently with astream of catalyst and, if desired, a stream of base and/or activator.lt is preferred to arrange the addition of catalyst, base and/oractivator such that addition thereof continues for a short period, i.e.,from about one-half to about 2 hours after addition of monomer iscompleted. During this period of unintemipted polymerization (socalledfinal stage digestion), it is generally preferred to maintain thetemperature of the resulting aqueous dispersion between about to about98 C to promote further conversion of monomer to polymer.

Before cooling, the aqueous dispersion of polymeric particles iscommonly rendered increasingly alkaline by adjusting the'pH to withinthe range of from about 6 to about 8. This may be done by adding ammoniaor a water-soluble amine or an inorganic base, such as potassium orammonium hydroxide, or a mixture thereof. Ammonium hydroxide, usuallygiving the best results in the least complicated way, is oftenpreferred.

Having permitted the alkaline, aqueous dispersion to cool to ambienttemperature, the aqueous dispersion of polymeric particles can beseparated from undesirable impurities such as coagulum by-product, byfiltering the aqueous dispersion of polymeric particles through astainless steel filter having the filter surface perforated tocorrespond with the standard 16 mesh size of the US. Standard SieveSeries.

The filtered aqueous dispersion of polymeric particles prepared by themethod described above and ordinarily containing from about 20 to about60 weight percent, preferably from about 40 to about 50 weight percent,of non-film forming polymeric solids which form the polymeric pigment ofthis invention is ready to be combined with a suitable binder to providea paper coating composition. dispersing the polymeric particles(pigment) of the aqueous dispersion in an aqueous medium containing thebinder or by blending the aqueous dispersion with the aqueous mediumcontaining the binder, thus eliminating the step of separating theparticles from the aqueous dispersion. Generally, suitable ratios ofbinder to polymeric pigment in the coating range from about 1.3 to about11.5 volume parts of binder, dry basis, to 38.8 volume parts, calculatedon a dry basis, of polymeric particles. The preferred ratios range fromabout 6.7 to about 10 volume parts of binder to 38.8 volume parts ofpolymeric particles.

lt is further understood that a combination of inorganic pigment andpolymeric pigment are contemplated in the practice of this invention. Insuch combination the polymeric pigment is used in an amount which iseffective to lower weight and/or to enhance the coating properties ofgloss, pick resistance, printing qualities, brightness or the like ascompared to a coating containing only the inorganic pigment as pigment.Preferably, the polymeric pigment constitutes at least about 1 weightpercent of the total pigment. Suitable inorganic pigments for thispurpose include clays such as kaolinite, illite, montmorillonite andbeidellite; and other materials such as titanium dioxide, kieselguhr,calcium carbonate, calcium sulfate, calcium sulfite, blanc fixe, satinwhite, and zinc pigments, e.g., zinc oxide, zinc sulfide and lithopane.

Binders suitable for the purposes of this invention include coatingcompositions which are non-solvents for the polymeric pigment to be usedand which are adaptable to a paper coating process to provide anadherent, smooth, glossy layer suitable for printing. Exemplary suitablebinders include the natural binders such as starch, modified starch, soybean, protein and casein and commonly known synthetic binders. Suitablemodified starch binders include oxidized, enzyme converted, orhydroxy-ethylated starch. Suitable synthetic binders include thestyrene/butadiene copolymer latexes; the latexes of polymers of alkylesters of a,B-ethylenically unsaturated carboxylic acids such as thealkyl acrylates and methacrylates, especially such aqueous dispersionsof polymers which include a small amount of a copolymerizedethylenically unsaturated carboxylic acid; the latexes of copolymers ofbutadiene and acrylonitrile, latexes of copolymers of vinyl acetate andthe alkyl acrylates, latexes of copolymers of butadiene and methylmethacrylate, latexes of copolymers of vinyl chloride and vinylidenechloride, latexes of vinyl chloride polymers, latexes of vinylidenechloride copolymers; aqueous dispersions of polybutadiene, polyvinylacetate, polyvinyl alcohol and other synthetic polymers commonly used aspigment binders in paper coatings. As is typical of such binders, theyare preferably film-forming at room temperature and must be filmformingat temperatures used in the paper coating operation.

The coating is applied to a paper substrate by a conventional techniquesuch as air knife, trailing blade, inverted blade, roll coaters,sprayers and the like.

After the coating is applied, the surface of the substrate is dried andoften is then finished by calendering or supercalendering. It iscritical that the temperature of the coating does not exceed thesoftening point of the polymeric particles, otherwise the particles willlose their discrete character or proper size and shape. When thisoccurs, the opacity and brightness of the coated surfaces dropssubstantially.

The use of polymeric pigment as at least a portion of the pigment in apaper coating compositions reduces the time required to dry coatings ofsuch compositions and improves finish and visual gloss to papers coatedwith such compositions.

Where the solvent sensitivity of a specific polymeric pigment presents aproblem, a coating containing the polymeric pigment may be applied tothe paper substrate as a base coating and such base coating issubsequently coated with a coating containing an inorganic pigment whichis not as sensitive to solvents. Since by that procedure muchlesscoating containing the inorganic pigment will be required to yield asurface with the desired gloss and opacity, the total coating weight isusually reduced about 25 per cent.

The following examples illustrate the invention, but are not to beconstrued as limiting its scope. Except as indicated, all parts andpercentages are by weight. Weight parts are based on 100 weight parts oftotal monomers unless otherwise indicated.

Example 1 Into a five-liter reaction vessel equipped with an agitator,reflux condenser, dropping funnels, thermometer. inert gas line,temperature control apparatus and temperature monitor is added 85 partsof water. After the water is heated to 90 C, an inert gas purge is begunand 0.5 parts of sodium persulfate is added. A continuous monomer streamconsisting of 97 percent styrene and 3 percent acrylic acid is begun atthe constant rate of addition of about 22.2 parts per hour.

After about thirty minutes and approximately 11.1 parts of the monomershave been added, the monomer feed is stopped, and eight parts of aseparate continuous addition stream consisting of 88 percent water and12 percent sodium persulfate is begun at a continuous rate of 1.45 partsper hour.

After about fifteen minutes, an exothermic condition of thepolymerization medium is observed to occur. At this time the monomerfeed is resumed at the previously specified rate. After about three morehours of continuous addition of the monomer and catalyst solu tionstreams, approximately 77.7 parts of the total 100 parts of monomershave been added to the reaction vessel. At this time, an emulsifiersolution consisting of 10 parts water, 2 parts sodium dodecyl diphenylether disulfonate (90 percent active), 4 parts octylphenyl polyethyoxyethanol (70 percent active) and 0.2 parts sodium persulfate is addedover a two to five minute period.

The monomer addition is continued at rate of about 22.2 parts per hourand is complete after about one more hour of continuous addition. Thecontinuous addition stream of catalyst and water is finished about onehour and a half after the monomer feed is complete. The reaction mixtureis digested for an additional hour at 82 C and cooled to ambienttemperature with stirring. The resulting aqueous dispersion contains48.3 percent polymer solids having an average particle diameter andscattering coefficient as shown in Table l.

Example 2 Into a reaction vessel as described in Example 1 is added 85parts of water. After the water is heated to 90 C, an inert gas purge isbegun and 0.5 part of sodium persulfate is added. A continuous monomerstream consisting of 92 parts styrene and 6 parts acrylonitrile is begunat a constant rate of addition of about 22.2 parts per hour.

At the same time as the monomer feed is begun, 2.0 parts of itaconicacid are also placed in the reaction vessel. After about thirty minutesof continuous monomer addition and approximately 1 LI parts of themonomers have been added, the monomer feed is then stopped and aseparate continuous addition catalyst solution stream consisting of 7parts water and 1 part sodium persulfate is begun at a continuous rateof about 1.45 parts per hour.

After about thirty minutes of digestion, the monomer feed is resumed ata rate of about 22.2 parts per hour. After about two more hours ofcontinuous addition of v the monomer and catalyst solution streams, 0.2part of ammonium hydroxide is added to the reaction vessel. After atotal monomer feed time of about three hours and forty minutes andapproximately 81.4 percent of the total monomers have been continuouslyadded to the reaction vessel, an emulsifier solution consisting of 10parts water and 3.3 parts aqueous solution of sodium lauryl sulfate (33percent active) is added over a two to five minute period.

The monomer addition is complete after about an additional fifty minutesof continuous addition. The catalyst solution stream is finished aboutone hour and a half after the monomer feed is complete. The aqueousdispersion is digested for an additional thirty minutes at C and cooledto room temperature with stirring.

The resulting aqueous dispersion contains 48.4 percent polymer solids,and has an average particle diameter and scattering coefficient(opacity) as shown in Table l.

Example 3 Into the reaction vessel described in Example 1 is added 125parts of water. The water is heated to 90 C under an inert gas purge,and 0.5 part of sodium persulfate is added. A continuous monomer streamconsisting of 97 parts t-butyl styrene and 3 parts acrylic acid is begunat a constant rate of addition of about 22.2 parts per hour.

After about thirty minutes of continuous monomer addition andapproximately 11.1 parts of the monomers have been added, the monomerfeed is stopped and a separate continuous addition catalyst additionstream consisting of 7 parts water and 1 part sodium persulfate is begunat a continuous rate of about 1.45 parts per hour.

After about twenty minutes, the monomer feed is resumed for anadditional one hour and ten minutes. At this time the latex is observedto be extremely viscous. The monomer feed is interrupted and thetemperature of the reaction medium is reduced to 50 C and maintaineduntil the viscosity of the latex is observed to decrease. Thetemperature is raised to 70 C and the monomer feed is resumed. Aboutforty minutes after the temperature is raised to 70 C, 0.2 part of 28percent ammonium hydroxide is added to the latex. After a total ofapproximately 76 percent of the monomers has been added, an emulsifiersolution consisting of 10 parts water and 1.5 part sodium lauryl sulfate(33 percent active) is added to the reaction vessel over a two to fiveminute period.

The monomer feed is complete after about one more hour of continuousaddition. The catalyst solution stream is finished about one hour afterthe monomer feed is completed. The aqueous dispersion is digested for anadditional 3 hours at 70 C, and cooled to room temperature withstirring. The resulting aqueous dispersion contains 39.4 percent polymersolids and has an average particle diameter and scattering coefficientas shown in Table l.

Example 4 Into the reaction vessel described in Example 1, is added 85.0parts of water. The water is heated to 90 C under an inert gas purge,and 0.5 part of sodium persulfate is added. A continuous monomer streamconsisting of 92 parts styrene, 5 parts divinyl benzene and 3 partsacrylic acid is begun at a constant rate of addition of about 22.2 partsper hour.

After about thirty minutes and approximately 11.1 parts of the monomermixture have been added, the monomer feed is stopped and a separatecontinuous addition stream consisting of 7 parts water and 1 part sodiumpersulfate is begun at a continuous rate of about 1.45 parts per hour.

After about thirty minutes, the continuous monomer addition is resumedat rate of about 22.2 parts per hour. After a total monomer feed time ofapproximately 3 hours and 35 minutes and approximately 79.6 percent ofthe monomers have been continuously added to the reaction vessel, anemulsifier solution consisting of 10 parts water, 1.5 parts aqueoussolution of sodium lauryl sulfate (33 percent active) and 0.2 partsodium persulfate is added to the reaction vessel over a two to fiveminute period.

The monomer feed is complete after about one hour and five minutes morecontinuous addition. The catalyst solution stream feed is completedabout one hour after the'monomer feed is completed. The aqueousdispersion is then cooled to C, digested approximately 2 hours andcooled to room temperature with stirring. The resulting aqueousdispersion contains 46.7 percent polymer solids and has an averageparticle diameter and scattering coefficient as shown in Table 1.Example 5 Essentially following the procedure of Exarnple l, 97 partsstyrene and 3 parts acrylic acid are polymerized at C by adding 90percent of total monomers prior to addition of emulsifier solutionconsisting of 2 parts sodium dodecyl diphenyl ether disulfonate (90percent active) and 2.8 parts octyl-phenylpolyethoxyethanol percentactive). The resulting aqueous dispersion contains 46.8 percent polymersolids and has an average particle diameter and scattering coefiicientas shown in Table 1. Example 6 Following the procedure of Example 4, amonomer stream consisting of 97 parts vinyl benzoate and 3 parts acrylicacid is polymerized at 75 C with addition of 90 percent of totalmonomers prior to addition of the emulsifier solution. The resultingaqueous dispersion contains 47.8 percent polymer solids and has anaverage particle diameter and scattering coefficient as shown in TableI.

Example 7 FollcTwing the procedure of Example 4, a monomer streamconsisting of 92 parts styrene, 5 parts alphamethyl styrene and 3 partsacrylic acid is polymerized. The resulting aqueous dispersion contains48.6 percent polymer solids and has an average particle diameter andscattering coefficient as shown in Table l. Example 8 Following theprocedure of Example 4, styrene as the monomer feed is polymerized. Theresulting aqueous dispersion contains 45.5 percent polymer solids andhas an average particle diameter and scattering coefficient as shown inTable 1.

Example 9 Following the procedure of Example 4, a monomer streamconsisting of 97 parts vinyl toluene and 3 parts acrylic acid ispolymerized. The resulting aqueous dispersion contains 47.7 percentpolymer solids and, has an average particle diameter and scatteringcoefficient as shown in Table I.

Example 11 Following substantially the procedure of Example 4 exceptthat at least autogenous pressure is employed, a monomer streamconsisting of 97 parts vinyl chloride and 3 parts acrylic acid ispolymerized at 75 C. The resulting aqueous dispersion contains 48.2percent polymer solids and has an average particle diameter andscattering coefficient as shown in Table l.

TABLE Average particle diameter micron Percent polymer solids ScatteringExample No. Latex polymer Stystyrene, AA-acrylic acid,VCN-acrylonitrile, lA-itaconic acid, TBS-bbutylstyrene, DVBdivinylbenzene, VBzvinyl benzoate, MSamethylstyrene. VTvinyltoluene, VCl-vinylchloride.

In determining scattering coefficient of a particular latex, the latexis combined with an ethylated starch binder composition in weight on drybasis of 38.5 weight parts (36.7 volume parts) of latex to weight parts(6.7 volume parts) of starch to form a coating composition. The coatingcomposition is applied with drawdown bars to a polished black glassplate in an amount sufficient to form a film having a wet thickness of1.5 mil and to a polished white glass plate in amount sufficient to forma film having a wet thickness of 3.0 mils and air dried at roomtemperature. Brightnesses for the coated black and coated white glassplates are determined by TAPPI Test Method T452-M58 and used incalculalion t i g wefiwiemesqsfilsisut i 3 2192 954 ,u i mthe equation:Scattering coefi'rcient=ST/220A wherein ST is a value read directly fromMittonJacobsen Graphs plotting the ST value against reflectance of thecoating over black and white glass plates and A is the weight in gramsof five square inches of coating. Reflectance is equal to 0.01 Xbrightness. The Mitton- Jacobsen Graphs are described in the OflicialDigest, September 1963, pp. 885 911 llot more than about 5 weightpercent OfpTartiCles have dfineters outside the range of 0.3 to 1.0microns.

Example 12 For purposes of further illustration of the invention, theprocedure of Example 1 is repeated several times with addition of theemulsifier solution at various times during polymerization. Theresulting aqueous dispersions contain 46-50 percent polymer solids andhave average particle diameters and scattering coefficients as shown inTable II. The percent yields of coagulum by-products formed in theseveral runs are also shown Not an example of this invention ND-Notdetermined Monomer-97% styrene and 3% acrylic acid Same as (2) in Tablel Example 13 For purposes of comparison, the procedure of Example 6 isrepeated several times with addition of the emulsifier solution atvarious times during polymerization. The resulting 46-50 percent polymersolids, aqueous dispersions have average particle diameters andscattering coefficients as shown in Table III.

TABLE Ill Scattering coefficient Monomer added Run prior to addition No.of emulsifier Average particle diameter micron Not an example of thisinvention "Monomer- 97% vinyl bcnzoatc, 3% acrylic acid Same 215(2) inTable l Example 14 For purposes of showing the effect of the presence oftrace amounts of emulsifier during polymerization of the initial portionof monomer, the procedure of Example 6 is repeated several times whereineach time a different very'small amountof emulsifier solution is addedat the start of the polymerization. The procedure is further altered byadding 67 percent of the monomers prior to addition of the specifiedmajor portion of emulsifier solution. The resulting aqueous dispersionscontain 46-50 percent polymer solids and have average particle diametersand scattering coefficients as shown in Table IV. Table IV Initialconcen- Run tration of Average particle Scattering No. emulsifierdiameter, micron coefi'rcient Not an example of this invention Weightpercent based on total monomer Same as (2) in Table l Example l Forpurposes of showing the effect of temperature in the process, theprocedure of Example I is repeated several times, each time at adifferent temperature. The resulting 46-50 percent polymer solids,aqueous dispersions have average particle diameters and scatteringcoefficients as shown in Table V.

TABLE v Run Polymerization Average Particle Scattering No. temperature,C. Diameter, micron Coefficient Same as (2) in Table l TABLE VI RunInitial stage Average particle Scattering No. digestion diameter, microncoefficient I No 0.52 0.20 Example No. I Yes 0.52 0.28

Same as (2) in Table I.

What is claimed is i l. A method for preparing an aqueous dispersion ofnon-film forming polymeric particles useful as pigment in paper coatingswhich comprises the steps of l subjecting a polymerization recipecomprising an aqueous medium, a catalyst and an incrementally-addedinitial portion of emulsion polymerizable monomer to conditions ofaqueous emulsion polymerization, essentially in the absence ofemulsifying agent, said initial portion constituting at least about 67weight percent up to and including 99.9 weight percent of total monomer,(2) subsequently adding emulsifying agent to the recipe in an amount offrom about 0.25 to about weight percent based on total monomer; and (3)continuing addition and emulsion polymerization of the remaining portionof the total monomer, said total monomer being capable of polymerizingunder said conditions of aqueous emulsion polymerization to form anon-film fonning, water-insoluble polymer, said emulsifying agentdefined as being selected from the group consisting of water-solubleanionic surfactant, nonionic surfactants and mixtures thereof, andexcluding polymerization catalysts and monomeric materials, said totalmonomer being polymerizable to form a non-film forming copolymercomprising from about 40 to about 99 weight percent of polymerizablehard monomer selected from the group consisting of monovinylidenecarbocyclic aromatic monomer, esters of a,B-ethylenically unsaturatedcarboxylic acids which polymerize to form nonfilm forming polymers,a,B-ethylenically unsaturated esters of non-polymerizable carboxylicacids which polymerize to form non-film forming polymers,afl-ethylenically unsaturated nitriles, other vinyl monomers whichpolymerize to form non-film forming polymers and mixtures of said hardmonomers; up to about 45 weight percent of an emulsion polymerizablesoft monomer selected from the group consisting of conjugated aliphaticdienes, alkyl acrylates, unsaturated esters of saturated carboxylicacids, esters and half esters of a,B-ethylenically unsaturatedpolycarboxylic acids, other vinyl monomers which polymerize to form filmforming polymers, and mixtures of said soft monomers, and from about 1to about 15 weight percent of at least one a,B-ethylenically unsaturatedcarboxylic acid.

2. The method according to claim 1 wherein the initial portion ofmonomer is from about 67 to about weight percent of total monomer.

3. The method according to claim 2 wherein step l) is interrupted afterabout 7 to about 15 weight percent of total monomer has been added andthe polymerization recipe is allowed to digest for a period in the rangeof from about 5 to about 60 minutes before addition of monomer isresumed.

4. The method according to claim I wherein the polymerization recipecontains less than about 0.015 weight percent of an emulsifying agentduring step l 5. The method according to claim 3 wherein thepolymerization recipe contains less than about 0.0 l 5 weight percent ofan emulsifying agent during step l 6. The method according to claim 1wherein total monomer polymerizes to form a non-film forming copolymercomprising from about 50 to about 99 weight percent of monovinylidenecarbocyclic aromatic monomer; from about 0 to about 49 weight percent ofemulsion polymerizable afi-ethylenically unsaturated nitrile and fromabout 1 to about 5 weight percent of emulsion polymerizablea,B-ethylenically unsaturated carboxylic acid.

7. An aqueous dispersion of non-film forming polymeric particles havingan average diameter in the range of from about 0.3 to about l.O micronwith no more than about 5 weight percent of said particles havingdiameters outside of said range, said dispersion being prepared by themethod of claim 1.

8. The aqueous dispersion according to claim 7 wherein the polymericparticles comprise from about 50 to about 99 weight percent ofmonovinylidene carbocyclic aromatic monomer; from about 0 to about 49weight percent of emulsion polymerizable afi-ethylenically unsaturatednitrile and from about 1 to about 5 weight percent of emulsionpolymerizable a,B-ethylenically unsaturated carboxylic acid.

9. An aqueous paper coating composition adapted to a paper coatingprocess for printing comprising a binder and a pigment wherein at leasta portion of the pigment is in the form of discrete, non-film formingpolymeric particles having an average diameter in the range of fromabout 0.3 to about 1.0 micron, with no more than 5 weight percent ofsaid particles having diameters outside of said range, said particlesbeing formed by the method of claim 1, said coating compositioncontaining from about 1.3 to about I 1.5 volume parts of binder per 38.8volume parts of pigment, said portion of polymeric particles beingsufficient in said 17 coating composition to produce a coating havingimproved properties, said particles being insoluble in water and thebinder.

10. A coated comprising a paper substrate having intimately adheredthereto an improved coating comprising a binder and a pigment, at leasta portion of said pigment being in the form of discrete, non-filmforming polymeric particles having an average diameter in the range offrom about 0.3 to about 1.0 micron, with no more than weight percent ofsaid particles having diameters outside of said range, said particlesbeing fonned by the method of claim 1, the volume ratio of binder topigment being in the range from about 1.3 to about I 1.5 volume parts ofbinder to 38.8 volume parts of pigment, said portion of polymericparticles being sufficient in said coating to impart improvedproperties.

11. The method of claim 1 wherein the emulsifying agent is present in anamount less than 0.015 weight percent based on total monomer duringpolymerization of the initial portion of the total monomer.

12. The method of claim 11 wherein said amounts of emulsifying agent isin the range from about 0.0001 to about 0.01 weight percent.

13. The method of claim 1 wherein after from about 7 to about 15 weightpercent of total monomer has been added to the resulting polymerization,the recipe is allowed to digest for about 5 to about 60 minutes beforeresuming addition of the initial portion of monomer.

/ UNITED STATES PATENT OFFICE (5/69) J a Q CERTIFICATE OF QQRREQTIONPatent No. 3,819,557 Dated June 25, 1974 Invmmr(s) Norman RaymondLoeffler and Hershel Beebe Prindle It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as's'hown below:

Column 8, line 35, before the word "dispersing", insert -The papercoating so described may be combined by--.

Column 14, line 58, the words"Table IV" should be center d above thetable.

Column 17, line 4, insert the word -paperbefore the word "comprising".

Signed and sealed this 5th day of November 1974.

(SEAL) Attest:

MCCOY M. GIBSON-JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

2. The method according to claim 1 wherein the initial portion ofmonomer is from about 67 to about 85 weight percent of total monomer. 3.The method according to claim 2 wherein step (1) is interrupted afterabout 7 to about 15 weight percent of total monomer has been added andthe polymerization recipe is allowed to digest for a period in the rangeof from about 5 to about 60 minutes before addition of monomer isresumed.
 4. The method according to claim 1 wherein the polymerizationrecipe contains less than about 0.015 weight percent of an emulsifyingagent during step (1).
 5. The method according to claim 3 wherein thepolymerization recipe contains less than about 0.015 weight percent ofan emulsifying agent during step (1).
 6. The method according to claim 1wherein total monomer polymerizes to form a non-film forming copolymercomprising from about 50 to about 99 weight percent of monovinylidenecarbocyclic aromatic monomer; from about 0 to about 49 weigHt percent ofemulsion polymerizable Alpha , Beta -ethylenically unsaturated nitrileand from about 1 to about 5 weight percent of emulsion polymerizableAlpha , Beta -ethylenically unsaturated carboxylic acid.
 7. An aqueousdispersion of non-film forming polymeric particles having an averagediameter in the range of from about 0.3 to about 1.0 micron with no morethan about 5 weight percent of said particles having diameters outsideof said range, said dispersion being prepared by the method of claim 1.8. The aqueous dispersion according to claim 7 wherein the polymericparticles comprise from about 50 to about 99 weight percent ofmonovinylidene carbocyclic aromatic monomer; from about 0 to about 49weight percent of emulsion polymerizable Alpha , Beta -ethylenicallyunsaturated nitrile and from about 1 to about 5 weight percent ofemulsion polymerizable Alpha , Beta -ethylenically unsaturatedcarboxylic acid.
 9. An aqueous paper coating composition adapted to apaper coating process for printing comprising a binder and a pigmentwherein at least a portion of the pigment is in the form of discrete,non-film forming polymeric particles having an average diameter in therange of from about 0.3 to about 1.0 micron, with no more than 5 weightpercent of said particles having diameters outside of said range, saidparticles being formed by the method of claim 1, said coatingcomposition containing from about 1.3 to about 11.5 volume parts ofbinder per 38.8 volume parts of pigment, said portion of polymericparticles being sufficient in said coating composition to produce acoating having improved properties, said particles being insoluble inwater and the binder.
 10. A coated comprising a paper substrate havingintimately adhered thereto an improved coating comprising a binder and apigment, at least a portion of said pigment being in the form ofdiscrete, non-film forming polymeric particles having an averagediameter in the range of from about 0.3 to about 1.0 micron, with nomore than 5 weight percent of said particles having diameters outside ofsaid range, said particles being formed by the method of claim 1, thevolume ratio of binder to pigment being in the range from about 1.3 toabout 11.5 volume parts of binder to 38.8 volume parts of pigment, saidportion of polymeric particles being sufficient in said coating toimpart improved properties.
 11. The method of claim 1 wherein theemulsifying agent is present in an amount less than 0.015 weight percentbased on total monomer during polymerization of the initial portion ofthe total monomer.
 12. The method of claim 11 wherein said amounts ofemulsifying agent is in the range from about 0.0001 to about 0.01 weightpercent.
 13. The method of claim 1 wherein after from about 7 to about15 weight percent of total monomer has been added to the resultingpolymerization, the recipe is allowed to digest for about 5 to about 60minutes before resuming addition of the initial portion of monomer.